Diary: Divine Mercy in My Soul is a book by Faustyna Kowalska, now a Roman Catholic saint. The book is based on the contents of her diary from 1925 (see paragraph 7 page 6) until her death in 1938.
The handwritten pages of Kowalska's diary turned into about 700 printed pages, published as the book Diary: Divine Mercy in My Soul. It reflects her thoughts, prayers and reported visions and conversations with Jesus on Divine Mercy. Kowalska's Vatican biography quotes some of her reported conversations with Jesus from her diary.